RDM, sync and timecode: the rest of the show traffic

Levels are the obvious thing Sig-Net carries, but a real lighting network does other jobs alongside the levels: it manages fixtures with RDM, it coordinates frame-perfect output with sync, and sometimes it runs the show on timecode. Sig-Net handles all three, and the way it handles them is worth a quick tour.

RDM, multi-controller-friendly

Sig-Net wraps standard E1.20 RDM packets inside its own envelope and routes them around the network using the same TUID-based addressing the rest of the protocol uses. A Manager fires an RDM command at a specific Node and a specific endpoint, the Node terminates or proxies the command, runs the actual RDM transaction, and sends back the response.

The interesting bit is what happens next. Every time a fixture's parameter state changes — whether triggered by an RDM command, by a local front-panel interaction, or by a queued message from a downstream responder — the gateway proactively multicasts a TID_RDM_RESPONSE containing the new value. Every Manager listening on the reply group instantly sees the update.

This is how Sig-Net does multi-controller RDM without a broker. RDMnet needed a central server because nobody trusted the network to keep multiple controllers in sync. Sig-Net just trusts the network: when something changes, everybody hears about it, and stale caches don't happen.

There's also a flow-control TID — TID_RDM_FLOW_CONTROL — that lets a gateway tell a Manager exactly how many slots in its RDM command queue are currently free. Consoles use this to burst RDM traffic right up to the gateway's limit without ever overflowing it.

V1.0 spells out the gateway's job more explicitly. A gateway is the sole RDM master on its downstream RS485 line: it runs its own continuous background GET QUEUED_MESSAGE loop on every discovered responder, it autonomously pumps ACK_OVERFLOW sequences (streaming the responses out as they arrive rather than batching), and it forwards ACK_TIMER responses to the network while quietly handling the physical timer locally. Manager applications must not try to drive any of that themselves over the network — the gateway has it covered.

Sync: frame-perfect output

The sync mechanism is opt-in per endpoint. When a Node receives a TID_LEVEL packet from a Sender, normally it applies the new data straight away. But if the same Sender also sends sync packets, the Node switches into synchronised mode for that endpoint: the level data goes into a holding buffer instead of straight to the output.

Then, when the matching sync packet arrives, every endpoint receiving from that Sender flips its buffer to the active output simultaneously. Fifty universes of LED tape can change look on the same network frame, with no tearing.

If sync packets stop arriving for 250 milliseconds, the endpoint flushes its buffer, falls back to asynchronous output, and waits. When sync resumes, it picks up cleanly. No restart, no manual intervention.

The Sender has to sit on the sync packet for at least 5 milliseconds after the last level packet so receivers have a moment to buffer everything, but that's the only timing constraint anyone has to think about.

Timecode: as many streams as you want

Timecode rides on Sig-Net using the same MIDI Timecode format as Art-Net, with one frame per network packet. It supports every common rate — 24, 25, 29.97, 30, 48, 50, 59.94, 60, 100, 119.88 and 120 fps — and you can run up to 255 independent timecode streams simultaneously on the same network. So the lighting console, the sound desk, the video server and the automation controller can each have their own timecode track if you wanted.

When the source is paused, it keeps transmitting the frozen value at 1 Hz so receivers don't drop the stream. If the stream actually does drop, receivers freeze on the last value and resume cleanly when a new packet arrives — they don't try to guess that time kept moving in the gap.

Next post: what happens when something breaks.

This series is based on v1.0 of the Sig-Net spec - visit Sig-Net.net for any updates.